1. Field of the Invention
The present invention relates to cold cathode fluorescent lamp (CCFL) driving systems for CCFL loads, and particularly to a driving system that powers on multiple CCFL loads from an off state to an operationally-on state.
2. General Background
Fluorescent lamps are typically used in a number of applications where artificial light is required but the power required to generate the light is limited. One such application is the backlighting for a notebook computer or similar portable electronic device. One popular type of fluorescent lamp is the cold cathode fluorescent lamp (CCFL), which is almost universally used in the panels of various LCDs (liquid crystal displays). The CCFL requires a high starting voltage (on the order of 700-1,600 volts) for a short period of time, to ionize the gas contained within the lamp tube and thereby ignite the lamp. After the gas in the CCFL is ionized and the lamp is lit, less voltage is needed to keep the lamp on.
CCFL tubes typically contain a gas, such as argon, xenon or the like, along with a small amount of mercury. After an initial ignition stage and the formation of plasma, electrical current flows through the tube, which results in the generation of ultraviolet light. The ultraviolet light in turn irradiates a phosphoric material coated on the inner wall of the tube, resulting in the emission of visible light. This process is achieved by the application of a driving system that can be utilized to generate an AC voltage to drive the CCFL load when a DC voltage is initially applied.
FIG. 3 shows a conventional CCFL driving system 10. The system 10 broadly includes a power supply 12, a CCFL driving circuit 14, a feedback loop 16, a controller 18, and a CCFL circuit 15. The feedback loop 16 includes sense impedance, such as a sense resistor Rs. The sense resistor Rs is configured for sensing the current Iout flowing through the CCFL loads included in the CCFL circuit 15, and further provides a feedback signal VFB as input to the controller 18. The CCFL driving circuit 14 is supplied a DC voltage Vcc by the power supply 12 and is controlled by the controller 18, and thus generates the AC voltage Vout applied to the CCFL loads. The controller 18 is further adapted to receive the feedback signal VFB from the feedback loop 16 so as to allow for the controller 18, through the CCFL driving circuit 14, to control the power delivered to the CCFL load. The CCFL driving circuit 14 generally includes a self-oscillating DC to AC converter, known as a Royer circuit, which commonly includes a single transformer 140 as shown in FIG. 3. Therefore, the DC voltage from the power supply 12 is converted into the AC voltage by the transformer 140, so that the AC voltage is provided to drive the CCFL loads included in the system 10.
One of the problems with the circuit shown in FIG. 3 is that one CCFL lamp may be ignited while the other one is still dormant. That is, not all of the lamps may be completely ignited. For example, since the sense resistor Rs merely senses the overall output current Iout, the CCFL driving system 10 may not detect the dormant CCFL lamp, and may continue to run normally. However, if either of the CCFL lamps is not ignited (i.e., stays dormant), the lighting of the entire large panel may be significantly affected. Furthermore, the dormant lamp may degrade the expected working lifetime of the other lamp, because each of the lamps bears much more current than previously. This is particularly so if either of the lamps is dormant on repeated occasions.
A so-called soft start mode is applicable to some CCFL driving systems disclosed in various articles and issued patents, such as, for example, in U.S. Pat. No. 6,501,234. The soft start mode applies to one CCFL load only, and is utilized to enable the CCFL load to be powered from an off state to an operationally-on state. The soft start mode has not been adapted to a system having a plurality of CCFLs, whereby each of the CCFLs can be completely ignited when the system is initially powered up.
What is needed, therefore, is a CCFL driving system which can be utilized to assure that each of CCFL loads included therein is powered from an off state to an operationally-on state.